A strain wave gearing with a built-in motor is provided with a motor, a wave gear mechanism enclosing the motor coaxially, and a heat-insulation spacing formed therebetween. The wave gear mechanism has a wave generator attached to the motor rotor so as to rotate integrally with the motor rotor. A wave generator plug of the wave generator is fixed to a rotor magnet back yoke of the motor rotor so as to enclose the rotor magnet back yoke. The spacing is formed in a contact surface portion between the rotor magnet back yoke and the wave generator back yoke, whereby heat transfer from the motor to the wave gear mechanism is suppressed.